(1) Field of the Invention
The present invention relates to a method for improving oil return in a refrigeration unit.
(2) Description of the Related Art
With reference to FIG. 1, there is illustrated a refrigeration unit 10. Typically, a refrigerant/oil mixture travels in a circuit from the compressor 11 through the condenser 13 to an expansion device 19 then to an evaporator 15 and then to suction line 21 and finally back to the compressor 11. In instances wherein an operating refrigeration unit 10 carries perishable goods at low ambient temperatures, the amount of cooling required to maintain the cargo at a given temperature is quite minimal. In such instances, the amount of refrigerant traveling through the coil of the evaporator 15 and suction line 21 is substantially reduced. Such reduction in the amount of refrigerant results in the retention and logging of oil in the evaporator. This logging of oil in turn compromises oil return to the compressor and thus reduces the heat transfer effectiveness of the evaporator coil or in some instances can even lead to complete oil pump out from the compressor resulting in the compressor failure.
One possible solution to reduce or eliminate the logging of oil in the evaporator 15 or suction line 21 would be to cycle the entire refrigeration unit 10. Under such a scenario, the refrigeration unit 10 is periodically turned on and off. If the unit is allowed to cycle in on and off fashion, then when the unit is in the “on” mode then the amount of refrigerant traveling through the system is higher than if the unit had to operate continuously to maintain the same temperature of the refrigerated space. Unfortunately, cycling the entire refrigeration unit 10 can result in a scenario wherein it is difficult to control the temperature of the space to be refrigerated within relatively narrow bounds. In addition, if the compressor does not run continuously there are additional cycling losses due to frequent start/stops resulting in increased power consumption. Also compressor reliability may be compromised due to frequent start/stops. Continual running of the compressor 11 on another hand allows for better maintenance of the temperature of the space to be refrigerated within relatively narrow bounds, and may also improve compressor reliability and operating unit efficiency.
What is therefore needed is a method for operating a refrigeration unit 10 on a continuous basis with sufficient amount of refrigerant traveling through the system to prevent logging of oil in the evaporator 15. Preferably such a method would involve the continual operation of the compressor 11.